The ability of the cerebral cortex to perform incredibly complex functions resides in its intricate neural circuits composed of a vast number of neurons. The synaptic interactions among cortical neurons ultimately manifest as the interplay between excitation and inhibition, two opposing forces that work together to orchestrate the spatiotemporal patterns of neuronal activity. Hence, the relationship between excitation and inhibition (E-I relationship) is fundamental to many functional properties of cortical neurons such as the orientation selectivity and contrast response function of visual cortical neurons. The importance of proper E-I relationship is also underscored by the discovery of altered E-I relationship in many neurodevelopmental and psychiatric disorders. However, the regulation of E-I relationship and the impacts of altering this relationship on the functional response properties of cortical neurons remain poorly understood. Thus, the overall goal of this project is to determine how the activity of individual neurons and homeostatic synaptic plasticity regulate cortical excitation, inhibition, and E-I relationship. To this end, we used the developing mouse visual cortex as a model system and developed molecular approaches to selectively reduce the excitability of a small number of layer 2/3 pyramidal neurons in vivo, such that we can determine the cell-autonomous effect of neuronal activity while minimizing the perturbation to the whole circuit. We found that these neurons counteract the activity perturbation by homeostatic changes at a specific subset of excitatory and inhibitory synapses. These results led to the central hypothesis that homeostatic plasticity differentially modifies distinct synaptic inputs of individual cortical neurons to regulate their E-I relationship, thereby maintaining the activity levels and functional response properties. We propose to combine molecular manipulations with optogenetic, physiological, imaging, and anatomical methods to systematically delineate the homeostatic changes at different synapses originating from distinct presynaptic neuronal types (Aim 1), to identify the underlying synaptic mechanisms of input-specific homeostatic plasticity (Aim 2), and to determine the impact of these synaptic changes on the visual response properties of neurons in vivo (Aim 3). The proposed research connects three levels of investigations from synapse to circuit to system. The successful completion of this project will provide insights into the role of homeostatic synaptic plasticity in regulating E-I relationship and functional response properties of cortical neurons. The outcomes will also have an impact on our understanding of how plasticity mechanisms help the brain cope with perturbations in general.